1. Field of the Invention
The present invention relates to solder alloys used for metal joining in electronic devices, more specifically to solder alloys which do not contain lead nor cause pollutions.
2. Description of the Related Art
In general, soldering is performed for the purpose of mechanical or electrical joining. When making soldering, the following points are required.
First, the solder alloys are required to be superior in joining properties and corrosion resistance.
Secondly, the solder alloys desirably have a high thermal fatigue strength and a desired soldering temperature, and do not contain lead from the environmental point of view.
That is, lead in any form shows an internal accumulative toxicity. Therefore, problems of air pollution and waste treatment in the lead smelting process, accumulation in the physical bodies of babies and pregnant women due to exposure to the air and contamination of foods and the like are concerned.
Thirdly, the solder alloys are required to be high in thermal fatigue strength. This is because, since semiconductor device chips generate heat when powered and the soldering part of joined chip metals is face joining, a large thermal strain generates in the soldering part of chips, and the solder alloys forming the soldering part are subjected to rigorous operation environment.
Fourthly, the solder alloys are desirably those which are high in melting points and hard to be affected by temperature profile of subsequent processes. This is because from the construction of semiconductor devices, solder alloys of a plurality of types with different soldering temperatures are used when a plurality of soldering steps are carried out in the production of semiconductor devices.
Conventional solder alloys include tin-lead (Sn--Pb) alloy, tin-silver (Sn--Ag) alloy, and tin-antimony (Sn--Sb) alloy. Features and problems of these alloys will be described in the following.
Since the tin-lead (Sn--Pb) alloy is low in tensile strength and superior in ductility, it is high in strain generation and low in fatigue strength. Consequently, as will be described below, in conjunction with its low heat resistance, it is low in thermal fatigue strength. The Sn--Pb alloy has an eutectic temperature of 183.degree. C. The melting point can be increased from 183.degree. C. to the vicinity of 300.degree. C. by increasing the Pb content. However, since this widens the solid-liquid coexistence area between liquid phase temperature and solid phase temperature (183.degree. C.) and the eutectic temperature is 183.degree. C., it has problems in that it is low in heat resistance and tends to undergo material degradation at relatively low temperatures. Further, it is not desirable as a solder alloy because it contains Pb. As solder alloys in place of the Sn--Pb alloy which do not contain Pb and are high in heat resistance, Sn--Sb alloy having a melting point of 232-245.degree. C. and Sn--Ag alloy having an eutectic temperature of 221.degree. C. are widely known.
The Sn--Ag alloy with the eutectic temperature of 221.degree. C. is good in thermal fatigue characteristics, however, from the practical point of view, in some cases it is required to be even further improved in thermal fatigue characteristic and have a higher melting point.
The Sn--Sb alloy is relatively higher in strength and is thus better than the Sn--Pb alloy. The Sn--Sb alloy contains 8.5% by weight of Sb, has a peritectic point at 245.degree. C., and Sb is used normally in an amount of less than 8% by weight. Since melting takes place between melting temperature 232.degree. C. of Sb and the peritectic temperature 245.degree. C., the solid-liquid coexistence area is small, the heat resistance is good, and one which is high in strength can be obtained by increasing the Sb content. However, the Sn--Sb alloy has problems in that it becomes degraded in processability when the Sb content is increased, and becomes low in wettability at soldering. Then, a solder alloy in which silver, copper, and nickel are added to the Sn--Sb alloy is known as one which is improved in thermal fatigue strength and wettability of the Sn--Sb alloy by suppressing the Sb content. However, since such an alloy contains tin as a main component, it has a problem in that when the solder alloy is melted an oxide film is formed on the surface and wettability or solderability is insufficient.
That is, in the Sn--Sb solder alloy, Sb is added to enhance the thermal fatigue characteristic at the melting point 232.degree. C. to 240.degree. C., and improvement of wettability and further increase in strength are achieved by the addition of Ag, Cu, and Ni.
Addition of Ag improves the fatigue strength and wettability. Ag exists in high concentration at crystal grain boundary, and suppresses movement of the crystal grain boundary thus improving the fatigue strength. However, the Sn--Ag alloy has an eutectic point (eutectic temperature) of 221.degree. C. at Sn-3.5 wt % Ag, and decreases in melting point by the addition of Ag, the decrease in melting point can be made up for by adding Cu and Ni to increase the melting point. Ag is added in an amount of 3 wt % and an alloy containing 6 wt % Ag has the same level of strength. When the Ag content exceeds 3.5 wt %, since the melting point (liquid phase temperature) is increased which requires an increase in the soldering temperature in order to ensure the wettability, and this further results in increased solid-liquid coexistence area.
Addition of Cu forms a solid solution in Sn and improves heat resistance and alloy strength without degrading the wettability. When Cu is added in an amount of more than 3 wt %, the melting point (liquid phase temperature) sharply increases. Further, as pointed out in Japanese Patent Application Laying-open No. 5-50286, formation of intermetallic compounds (Cu.sub.3 Sn and the like) increases, resulting in degraded fatigue strength. Even 0.5 wt % addition of Cu improves the strength.
Addition of Ni, since which is high in melting point (1450.degree. C.), provides thermal stability of the alloy, formation of fine crystal texture, improvement of thermal fatigue characteristic by formation of a Ni--Sn compound, and suppresses formation of intermetallic compound (Cu.sub.3 Sn) which decreases joining strength when soldered with a Cu substrate. When the Ni content is increased (more than 5 wt %), alloy production becomes difficult, and viscosity becomes high at soldering which decreases solder spreading. When the Ni content is less than 1.0 wt %, the strength and wettability are improved. When the Ni content exceeds 1 wt %, the resulting alloy becomes hard and rolling processability is impaired.